The self administration by inhalation of a pharmacologically active compound is becoming increasingly common in the treatment of various conditions, particularly respiratory problems such as asthma. Such a compound can be provided in a finely divided particulate form in a dispenser, commonly referred to as an inhaler.
The user of such a device self administers a dose of the compound by inhaling through a mouthpiece. This sets up a flow of air which ejects the dose of particulate material from the device. An example of such a device is shown in European Patent specification No. EP 0211595 (Glaxo Group Limited), and has a disc-shaped blister pack for containing a number of individually encapsulated doses of dry powder medicament, and a powder dilutant such as lactose. Numerous other designs of inhaler have been proposed, for example, the inhaler shown in U.S. Pat. No. 5,301,666, in which the doses of particulate material are contained in compartments in a series of rings that are rotated and moved axially to bring each compartment in turn into registry with an airway in the device.
The volume and flow rate of air inhaled through an inhaler may vary considerably from one use to another, particularly if the user has a respiratory complaint, the symptoms of which may vary in their severity.
The variation in rate of flow and/or volume of air inhaled can cause variations in the amounts of powder ejected from the device. The effect that this variation has on the amount of pharmacologically active compound which is actually administered each time the device is used is small if the device stores the compound mixed together with a relatively large amount of dilutant. However, it is desirable for the compound to be stored with relatively little or no dilutant, as more doses of material can be stored in an inhaler of a given size if the compound is relatively concentrated. In addition, the administering of a dose of medicament with little or no dilutant requires the user to inhale a smaller volume of powder, which is less likely to leave an unpleasant sensation in the user's mouth and throat.
However, in such cases, the variations in the amount of powder ejected from the inhaler can cause significant variations in the amount of dose actually administered to the user.